1. Field of the Invention
The present invention relates to image processing techniques for processing an image signal output from a solid-state imager such as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor, and in particular for processing an image signal output from a solid-state imager having a color filter array.
2. Description of the Related Art
Digital color imaging devices such as digital still cameras and digital video cameras with CCD or CMOS image sensors are now in common use. Many of them use single plane solid-state imagers having a single image plane that receives a focused optical image through a single color filter array. A Bayer arrangement of primary colors or secondary colors is widely used in the color filter array. A Bayer array of primary colors is generally an array of red (R), green (G), and blue (B) color filters; a Bayer array of complementary colors is, for example, an array of cyan (C), magenta (M), and yellow (Y) color filters.
Methods of improving the sensitivity of a color imaging device include electrically amplifying the output signal of the solid-state imager and lengthening the exposure time by lengthening the charge storage time of the solid-state imager. An image signal captured in a dim environment, however, includes a comparatively large noise component, so if it is electrically amplified, or if a long exposure is made, the S/N ratio (signal-to-noise ratio) of the captured image may drop. To achieve both high sensitivity and a high S/N ratio, a digital pixel addition method has been proposed by Ishii in, for example, Japanese Patent Application Publication No. 2000-184274. In this proposed method a number of pixels of the same color are selected and their digital signals are added.
A problem with the proposed digital pixel addition method is that since a single pixel is generated by adding the digital signals of a number of pixels (for example, two or three pixels), resolution is lost and the image quality is impaired.